The present invention relates to a compression bellows, particularly for use in compressing a solid oxide fuel cell stack.
Solid oxide fuel cells (SOFCs) are constructed by stacking a series of flat interconnect plates and planar fuel cells together to form a fuel cell stack. Gaskets are placed between each interconnect plate to seal against the leakage of the gases utilized by the fuel cells. The gaskets require a constant clamping force to be exerted upon the stack to ensure proper sealing. As the fuel cell stack temperature increases from ambient to an elevated operating temperature, the different components comprising the stack and the clamping mechanism thermally expand at different rates. The net result can be a decrease in the clamping force on the gaskets, leading to gas leakage from the stack.
Conventional stack compression methods all have disadvantages for use with planar SOFCs. Most prior art compression methods utilize some form of threaded metal rods, which are tightened between the top and bottom plates of the SOFC stack by the use of nuts or other means. These metal rods lose strength and tend to creep at the elevated temperature encountered in fuel cell operation, reducing the amount of compression applied to the stack. Another disadvantage of the threaded rods is that they must be individually tightened to the correct amount of pre-load on the stack. If the correct amount of pre-load is exceeded there is a possibility of cracking the ceramic cells inside the stack, and if undetected this can result in burning of the hydrogen fuel, and catastrophic failure of the stack. A further disadvantage of the prior art is that the overall height of the stack may vary through thermal cycling, as the seals have a tendency to shrink. This will result in reduced compression on the stack, and the possibility of gas leakage. It is not practical to retorque the rods after each thermal cycle.
Therefore, there is a need in the art for a compression system suitable for use in a SOFC stack which mitigates the difficulties found in the prior art.
The present invention is directed to a compression system suitable for use in an SOFC stack, and is tolerant of thermal cycling. The compression bellows of the present invention may also find application in other high-temperature configurations where a compressive force at elevated temperatures ( greater than 500 deg. C.) is required.
Therefore, in one aspect, the invention may comprise a solid oxide fuel cell stack comprising a plurality of planar fuel cells, interconnects and gasket seals, a top plate and a bottom plate, and a compression apparatus disposed between the stack and at least one of the top or bottom plates, said compression apparatus comprising a shell defining a hermetically sealed interior volume, said shell applying a compressive force to the stack in response to an increase in temperature. The compression apparatus, or bellows, is placed against the fuel cell stack, between top and bottom plates which are fixed together with tie rods. At the operating temperatures of the fuel cells, the bellows applies a compressive force to the stack, by expanding or tending to expand against the stack.
In another aspect, the invention may comprise a solid oxide fuel cell stack compression apparatus comprising a cylindrical shell defining a hermetically sealed interior volume, said shell having a vertical dimension and a radial dimension, wherein the interior volume of the shell comprises a fluid which expands in volume and/or pressure in response to an increase in temperature, and wherein the shell may expand vertically but not radially.
In accordance with another aspect, the invention may comprise a compression apparatus for SOFC stacks comprising a shell defining a hermetically sealed interior volume filled with an expansion fluid, said shell having a vertical dimension and a horizontal dimension, wherein the shell is deformable in the vertical dimension as a result of an increase in pressure and/or volume of the expansion fluid.